bug in substitute-env-vars

[nivpgir]

I belive I found a bug in substitute-env-vars:
reproduction:
running the following command
(display (substitute-env-vars "$SHELL bla bla bla"))

output:

assertion-violation: argument of wrong type [string-length]
                     #f

expected:

"/bin/bash bla bla bla"

this won't happen if the given string is the variable name by itself, only if the string is a space separated words and a '$ sign in front of one of them.

I've looked at the implementation substitute-env-vars, and a fix could be changing '#/' in the end of the function with a '#\ '.
that at least works for me.

all of the above is valid, of course, if the function should be able to perform on strings which include multiple words... if not, I think it should at least be noted on the documentation.